Recently, many types of radar apparatuses using microwaves and millimeter waves have been proposed. While the application range of the radar apparatuses is diversified, if the radar apparatus is mounted on a vehicle, not only a distance between a preceding or following vehicle and the vehicle with the radar apparatus can be accurately detected, but also an obstacle present at a position in a blind spot of a driver, for example, an obstacle located diagonally backward or near a bumper can be detected to warn the driver, or an unavoidable collision closing on is detected to perform a pre-crash control, such as fastening a seat belt.
When such a radar apparatus is mounted in many vehicles, a problem of interference between the radar apparatuses occurs. That is, when vehicles having the radar apparatus mounted thereon approach each other, or when a plurality of radar apparatuses is mounted on the same vehicle, since the distance between the radar apparatuses is short, a strong interference occurs. As a result, accurate measurement cannot be performed.
As a method of preventing the interference between the radar apparatuses, for example, a method disclosed in Japanese Patent Application Laid-Open No. H7-318639 is known. In the method described in the above application, an interference preventing apparatus that receives an interference wave, inverts the phase thereof, and retransmits the wave is installed. However, to prevent interference over a wide range, there is a problem that many interference preventing apparatuses must be installed, as well as a problem that interference cannot be prevented at all in an area where the interference preventing apparatus is not installed. Furthermore, in the case of vehicle radar, the position between the radar apparatuses is not always constant, and hence, it is difficult to solve the problem of interference with the method described in the above publication.